function timeout10(params) {
    return new Promise(resolve => {
        setTimeout(() => {
            resolve(params);
        }, 10);
    });
};
let a = '';
let b = '';
let c = '';
timeout10('hello')
    .then(res => {
        a = res;
        return timeout10('lagou');
    })
    .then(res => {
        b = res;
        return timeout10('i ❤ u');
    })
    .then(res => {
        c = res;
        console.log('res', a + b + c);
    });
